In a two-stroke internal combustion engine used for a motorcycle or an outboard motor of a ship, usually a mixture of gasoline and oil is used as the fuel. However, in a motorcycle driven a cold area in which a snow-removal agent is spread on a road, or in an outboard motor exposed to seawater, air containing salt from the surrounding atmosphere is introduced from a carburetor to the crankcase in addition to the mixture of gasoline and oil, so that a supporting bearing for jointing within the high-temperature crankcase can be easily corroded. Further, in the case of the motorcycle, because the oil in the exhausted gas released from the exhaust pipe (muffler) can attach on and dirty the back of the rider, in recent years, the proportion of the oil is reduced so that the ratio between the gasoline and oil in the mixture is changed from the conventional range of 15:1 to 50:1 to a ratio of 200:1. However, reducing the proportion of oil severely affects the lubrication of the bearing. If the bearing is rotated at a high speed of 10,000 to 30,000 rpm at a high temperature of 150.degree. C. through 200.degree. C., it may be easily burnt out to thereby shorten the life time of the bearing or the life time of the engine.
On the other hand, in accordance with the high performance of the engine in recent years, the roller bearing in the joint structure between the crankshaft and the connecting rod requires a high performance. That is, when the bearing is rotated at a high speed at a high temperature, a PV value is required to be reduced. The bearing is required to be extremely hard.
However, at the present time, any joint structure meeting with the above requirements to resolve the problems has not been proposed. In the meantime, in a particular field of industry such as the machine tool industry, to make a high performance bearing, a cylinder roller bearing and a ball bearing are provided with a rotator made of a ceramic material as proposed. The ceramic is usually hard as compared with a bearing steel, but it has inferior toughness. Therefore, conventionally, the ceramic can be used in practice as a rotator of a ball bearing, or a roller bearing whose length is about 1.5 times the length of the diameter. However, the shape in which a needle roller is formed does not have good sintering properties. In particular, no needle roller has been proposed which has a corrosion resistance suitable for driving under the severe conditions present in the crankcase of an engine.
Incidentially, a roller used in a needle roller bearing is defined to be Da.ltoreq.5mm, 3Da.ltoreq.l.ltoreq.10DA under the Japanese Industrial Standard (JIS) B1506 where the diameter is Da and the length is l.